Railroad crossing warning systems are perhaps the most familiar of a variety of vehicle collision/crossing warning systems. The purpose of such warning systems is to notify vehicles and/or stationery warning indicators of the approach and/or proximity of a vehicle. Other examples of such warning systems include emergency vehicle traffic light override systems, automobile navigation systems, airport and construction zone vehicle tracking systems and other navigational control and warning systems.
Because of the safety importance of vehicle collision/crossing warning systems, reliability and failure free operation are critical requirements in the design of such a system. In order to meet these design requirements, most existing vehicle collision/crossing warning systems are relatively expensive and require some form of centralized or coordinated communication scheme among the vehicles and other components that are part of the warning system. In the case of stationery warning components, such as railroad crossing warning systems or traffic light intersections systems, installation of such warning systems can require significant effort and usually involves providing power and communication wiring as part of the installation.
Traditional railroad crossing warning systems, for example, have relied on the railroad tracks themselves to detect an approaching locomotive and activate a warning signal apparatus. As the wheels of an approaching locomotive pass by a detector positioned at a predetermined location along the tracks relative to the crossing, the detector senses an electrical short across the tracks and sends a signal to a controller that activates flashing lights and/or descending gates at the crossing. The expense of installing such a traditional railroad crossing warning system, coupled with the requirement for AC electrical power to operate the warning system, have limited the use of such warning systems to urban areas and other high volume traffic crossings.
One alternative to such hardwired collision/crossing warning systems involves the use of wireless transmitters and receivers. U.S. Pat. Nos. 4,723,737, 4,942,395, 5,098,044, 5,739,768 and 6,179,252 are examples of such systems. Another alternative involves the use of global positioning satellite (GPS) technology to identify the location and movement of vehicles within the system. Examples of warning systems that utilize GPS technology are described in U.S. Pat. Nos. 5,325,302, 5,450,329, 5,539,398, 5,554,982, 5,574,469, 5,620,155, 5,699,986, 5,757,291, 5,872,526, 5,900,825, 5,983,161, 6,160,493, 6,185,504 and 6,218,961, as well as PCT Publication Nos. WO9909429 and WO101587 and Japanese Abst. No. JP11059419. Generally, these alternatives rely on some type of centralized or coordinated communication scheme to keep track of multiple vehicles and components or to confirm transmission of messages between vehicles and components within the warning system.
Despite these developments, there continues to be a need for a relatively inexpensive, low-power vehicle collision/crossing warning system that enables simple and decentralized installation, operation, and maintenance of a reliable vehicle collision/crossing warning system.